Due to their superior resistivity characteristics, magnetoresistive layer systems according to the spin valve principle offer interesting application possibilities, in particular in motor vehicles, for sensing an angle, a current intensity, or a rotational speed. For this purpose, the layer systems usually have four magnetoresistive layer systems interconnected as a Wheatstone bridge circuit, at least one of which is an active magnetoresistive layer system, i.e., a layer system operating on the basis of the GMR (giant magnetoresistance) or AMR (anisotropic magnetoresistance) effect. The individual magnetoresistive layer systems in such a layer system are furthermore usually designed to be serpentine-shaped in top view having a square or rectangular bottom surface and have a sequence of thin layers forming a “spin valve,” i.e., a layer sequence having an antiferromagnetic layer, a first ferromagnetic layer, a non-magnetic layer, and a second ferromagnetic layer.
Due to the “exchange bias effect” appearing at the interface between the two layers, such a spin valve having an antiferromagnetic layer and an adjacent first ferromagnetic layer shows a magnetic hysteresis curve shifted by the “exchange bias field.” The structure of a magnetoresistive layer system in the form of a spin valve and its use as a magnetoresistive sensor element has been described in detail, for example, in German Published Patent Application No. 198 43 348, which describes, in particular, the layer structure, the function of the individual layers and the mode of operation of such a sensor element.
To produce a Wheatstone bridge circuit having four magnetoresistive layer systems, for example, it is necessary to be able to define and independently adjust each of the directions of the resulting magnetization in the first ferromagnetic layers adjacent to the antiferromagnetic layers in each of the bridge branches of the Wheatstone bridge. This is accomplished, for example, by heating and subsequently cooling the magnetoresistive layer system and in particular the antiferromagnetic layer integrated therein over a material-specific “blocking temperature,” or threshold temperature, in an externally applied magnetic field. This procedure is explained, for example, in PCT Publication No. 00/79298, the magnetoresistive layer system being heated to adjust the resulting direction of magnetization by inputting heat using a laser pulse or by applying an electric current to the layer system.
German Published Patent Application No. 198 30 344 describes another method of adjusting the magnetization of a bias layer of a magnetoresistive sensor element, the bias layer being part of an AAF system (artificial antiferromagnetic system), which includes a bias layer, a flux guiding layer, and a coupling layer situated between the two which antiferromagnetically couples the two layers. In the method described there, the sensor element is initially heated to a predefined temperature using pulsating currents conducted through the sensor element; subsequently a magnetic adjusting field is applied; then the adjusting field is switched off after a predefined period of time, and finally the sensor element is cooled to the initial temperature. German Published Patent Application No. 198 30 344 also explains once more in detail the interconnection of a plurality of magnetoresistive layer systems to form a Wheatstone bridge circuit having different resulting directions of magnetization in the individual layer systems. In addition, this document contains detailed information regarding the layer structure and the composition of the individual layers of the magnetoresistive layer system.
Finally, German Published Patent Application No. 198 43 350 describes an electronic component, in particular a chip element, which has at least one magnetoresistive element situated on a substrate and having a sensor function, and at least one magnetoresistive element situated on a substrate and having a memory function. This document also describes that the directions of magnetization of the individual layers of the elements may be oriented parallel or antiparallel to one another by applying a current to printed conductors running underneath or above the magnetoresistive layer systems or elements.